1. Field of the Invention
The present invention relates to the detection of water constituents and particularly to the measurement of water salinity especially in a stream comprising a mixture of oil, water and gas.
A flowing fluid mixture of oil, water and gas is a common occurrence in the oil industry being a product of an unprocessed well stream. Such a flow mixture is referred to as a multiphase flow in which the oil, water and gas are considered as different phases. It is often required to know characteristics of individual phases within such a multiphase fluid flow for example the salinity of the water phase, and to measure these characteristics without separating the phases.
The salinity of the water phase can affect the accuracy of sensors and measurement methods applied for multiphase measurements It is required therefore to accurately determine the water phase salinity and to use the measurement to calibrate the other sensors or measuring equipment. Alternatively the water salinity is often of interest to the well operators as an absolute value which may then be compared with other wells or monitored over a period of time.
2. Description of the Related Art
Water phase salinity can be calculated according to the method described in EP 0 236 623. This prior art teaches irradiating the multiphase fluid flow with radiation at four distinct energy levels and taking measurements of the incident and transmitted radiation to do calculations involving at least radiation measurements of the four distinct energy levels. This known method requires at least two different radiation isotopes and has the disadvantage of being very sensitive to errors in the measured oil, water and gas path lengths and densities. It is also expensive to use two isotopes and to all intents and purposes this prior art does not provide a satisfactory method of measuring salinity in the water phase in a practical situation. It is only accurate and suitable for controlled laboratory conditions.